1. Field of the Invention
The present invention relates to an electric power steering system employing an electric motor as a source of a steering assist force.
2. Description of Related Art
Electric power steering systems for motor vehicles are generally adapted to control the output rotation of an electric motor according to a steering torque applied to a steering member such as a steering wheel when the steering member is rotated for operation. The rotation of the electric motor is transmitted to a steering mechanism via a reduction gear mechanism to assist the operation of the steering mechanism according to the rotative operation of the steering member, whereby a steering effort of a driver can be alleviated.
A combination of a worm and a worm wheel is provided as the reduction gear mechanism. A worm shaft provided with the worm is rotatably supported with longitudinally opposite ends thereof held by a pair of bearings. However, noises are often caused by rattle (or play) due to internal clearances of the bearings.
Therefore, a pair of ball bearings are employed as the bearings, and a preload is applied longitudinally thereto for elimination of the longitudinal internal clearances. However, the use of the two ball bearings results in higher costs.
On the other hand, a mechanism for elimination of backlash between the worm shaft and the worm wheel is employed, which is adapted to bias one end of the worm shaft radially toward the worm wheel. In this case, the one end of the worm shaft is radially movable. Even with the use of a ball bearing, this makes it difficult to apply a preload to the ball bearing.